BY JEFF DARLINGTONjdarlington@MiamiHerald.comIt hasn't been a good week for a very important competition that will decide to the Dolphins' kick returner for 2010.

Three days after running back Kory Sheets tore his Achilles during Wednesday's practice, running back Patrick Cobbs has been sidelined with an injured hamstring, according to a source

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The team doesn't yet know when Cobbs will be able to return - whether it will only take a few days to heal or longer - so the situation isn't yet necessarily dire. It could simply turn into a minor setback.